1. Field of the Invention
The present invention relates to imaging devices and imaging methods for shooting a moving image and recording image data of the moving image.
2. Description of Related Art
Conventionally, an imaging device has an AE (Auto Exposure) processing function, calculates an appropriate exposure amount based on a brightness component of image data, and controls a shutter speed, an aperture, an ISO sensitivity, and the like. At the time of moving image shooting, the exposure time (shutter speed) of each frame is controlled by a so-called electronic shutter. Therefore, in controlling so as to achieve appropriate exposure at a shutter speed, if the brightness of a subject is high, the electronic shutter speed becomes shorter than the frame rate of a moving image. In this case, the reproduced moving image produces a flipping feeling (the feeling that the image instantaneously changes and awkwardly moves, as in a flip comic), resulting in an unnatural moving image instead of showing a smooth movement.
Then, an imaging device has been proposed for reducing the shutter speed and edge enhancement in the case of a moving subject while increasing the shutter speed and edge enhancement in the case of a static subject (see Japanese Patent Laid-Open No. 2007-189295 (Patent Literature 1)).
At the time of moving image shooting, the moving image needs to be recorded in accordance with the frame rate in reproducing the moving image. For example, in the case of a full high definition television (2K moving image), there are the standards of 60p and 30p, in which 60 frames and 30 frames are shot per second, respectively. There is a method for changing the aperture, gain, and ND filter in accordance with the shutter speed in order to make the lightness of an image constant.
In the case of a moving subject, unless the shutter speed is basically adjusted to the recording frame rate (e.g., the shutter speed is adjusted to 1/30 seconds in the case of HD 30P), all of the movements of the subject within a time period corresponding to one frame cannot be recorded. That is, the shutter speed is determined in accordance with the recording standard, and therefore as described in Patent Literature 1, the flipping feeling cannot be eliminated at all even if the shutter speed is imperfectly changed in accordance with the brightness of the subject.
Moreover, there can be contemplated a method for adjusting the shutter speed in accordance with the recording frame rate, and putting in/out, at high speed, an ND filter (dimming filter) to/from the optical path of a shooting optical system in response to a change in the brightness of a subject, and a method for quickly varying the aperture. However, such methods make human eyes feel blinking and therefore will degrade the quality of an image in more sophisticated moving image shooting.
Moreover, a method for changing the gain can be contemplated. In this case, if a subject becomes lighter, the gain tends to be reduced, and therefore even if the gain is reduced to 0 dB or less and when the image signal from an image sensor saturates, the linearity of lightness cannot be secured. This problem can be solved by slowly moving the aperture. However, taking into consideration that for still image shooting an aperture which can be moved at high speed is needed, individual apertures have to be provided for still image shooting and for moving image shooting, resulting in an increase in size of the device. Moreover, a small device without an aperture mechanism has to be controlled using an ND filter and thus the quality of an image will degrade as described above.